This invention relates to a motor driving apparatus which detects zero-crossing of back electromotive forces generated in a non-activation phase (open phase) in windings to perform position sensorless PWM driving of the motor, in particular, a starting control method in the motor driving apparatus.
In recent years, a brushless motor has been generally employed to drive a recording medium such as a hard disk and an optical disk, or a fan, a compressor and the like in air conditioning equipment. For a wide range of variable speed control or reduction in electricity consumption, such brushless motor is driven by PWM operation by using an inverter device. In the brushless motor having three-phase windings, to detect position of a magnetic pole of the rotor, position sensors such as Hall devices are generally arranged every 120 degrees of electrical angle. In opposition to the brushless motor having such position sensors, for the purpose of reducing costs and size, there is a demand for a brushless motor without position sensor and various sensorless driving techniques have been developed. Means of realizing sensorless driving include a method of performing 120-degree activation in electrical angle and detecting zero-crossing of the back electromotive forces generated in the non-activation phase (open phase). 120-degree activation means that the width of activation has an electrical angle of 120 degrees.
FIG. 22 is a block diagram showing a conventional motor driving apparatus which performs sensorless driving. In FIG. 22, a motor 10 is comprised of a rotor having a field part formed of a permanent magnet (not shown) and a stator in which three-phase windings 11, 12 and 13 are Y-connected. An outputting part 120 is disposed between a power supply 1 and the ground (GND) and bridge configuration is achieved by three high-side power transistors and three low-side power transistors to supply electric power to the three-phase windings 11, 12 and 13. By comparing three-phase terminal voltages Vu, Vv, and Vw with a center tap voltage Vc, a position detector 130 detects the position of the rotor and outputs position detection signals UN, VN and WN to an activation controller 140 comprised of a CPU. The activation controller 140 outputs high-side activation control signals UU, VU and WU and low-side activation control signals UL, VL, and WL for controlling each power transistor of the outputting part 120 according to the position detection signals UN, VN and WN and performs activation timing control of the three-phase windings 11, 12 and 13. Sensorless driving of the motor 10 is performed by the motor driving apparatus thus configured.
FIG. 23 is a timing chart showing operation of each part of the motor driving apparatus having the configuration as shown in FIG. 22. In FIG. 23, waveforms Eu, Ev and Ew are waveforms of the three-phase back electromotive forces and waveforms UN, VN and WN represent the position detection signals. The position detection signals UN, VN and WN are results obtained by comparing the three-phase terminal voltages Vu, Vv, and Vw with the center tap voltage Vc, respectively, each having an edge at zero-crossing points of the waveforms of the three-phase back electromotive forces Eu, Ev and Ew, respectively. Waveforms UU and UL are the high-side control signal and the low-side control signal of the U-phase upper and lower power transistors in the outputting part 120. A waveform Iu is a drive current flowing to the U-phase winding.
The activation controller 140 measures rotor rotating speed from rotor position detected by the position detection signals UN, VN and WN and rate of change in speed and calculates a delay time corresponding to an electrical angle of 30 degrees. Activation control is performed at the timing according to this delay time to output the high-side activation control signals UU, VU and WU and the low-side activation control signals UL, VL and WL. By controlling activation timing in this manner, the conventional motor driving apparatus performs motor driving control.
The motor driving apparatus which calculates the delay time corresponding to an electrical angle of 30 degrees on the basis of the position detection signals UN, VN and WN and performs activation control at the timing according to the calculated delay time as described above is disclosed in, for example, an Official Gazette of Japanese Patent Publication No. 2778816.
FIG. 24 is a block diagram showing a configuration of another conventional motor driving apparatus. The activation controller 140 formed of a CPU comprises a controller 140A and an operating part 140B. Outputs of the controller 140A and the operating part 140B are switched by a switch of a switching part 150 and sent to the outputting part 120 as activation control signals. The outputting part 120 supplies electric power to the motor 10 according to the activation control signals. The position detector 130 detects a rotor position and outputs the position detection signals indicating the rotor position to the operating part 140B. The outputs of the controller 140A and the operating part 140B are input also to a comparing circuit 160, and a phase difference between both signals is detected and output to the controller 140A and the switching part 150.
Operation of the motor driving apparatus thus constituted shown in FIG. 24 will be briefly described.
The controller 140A outputs a synchronization signal for performing synchronous operation, and accordingly the motor 10 is started to perform synchronous operation. When the rotor starts rotating, the position detector 130 detects rotor position by back electromotive forces and the operating part 140B outputs the activation control signal according to the position detection signals. The comparing circuit 160 detects a phase difference between the synchronization signal of the controller 140A and the activation control signal of the operating part 140B, and when the phase difference falls within a predetermined value, the switch of the switching part 150 is switched from a terminal 150A to a terminal 150B. That is, synchronous operation is switched to position detection driving. A phase difference detection signal is fed back to the controller 140A and the synchronization signal of the controller 140A is corrected so that phase difference may fall within the predetermined value in the comparing circuit 160.
As described above, the conventional motor driving apparatus in which the phase difference between the synchronization signal and the activation control signal according to the position detection signal is detected, and at the time when the phase difference falls within the predetermined value, synchronous operation is switched to position detection driving, is disclosed in, for example, the Official Gazette of Japanese Unexamined Patent Publication No. Hei 7-87783.
In the conventional motor driving apparatus thus configured, sensorless driving is performed by generating the activation control signals on the basis of the position detection signals using back electromotive forces. In such a method, when the rotor rotating speed is slow, for example, at starting, the position detection signals include many detection errors since back electromotive forces is small, etc. That is, since displacement in the detected phase arises with respect to the real phase which should be detected, there is a problem that a starting failure such as oscillation, loss of synchronism and reverse rotation may be caused depending on the initial rotor position, or even if the rotor rotates in the normal direction, sufficient starting torque cannot be obtained, thereby making starting time longer. Especially in driving of the motor for information equipment such as an optical disk, reduction in starting time has been strongly desired and stable sensorless starting in sensorless driving is an absolute requirement.
To solve the above-mentioned problem, the present invention intends to provide a motor driving apparatus which can carry out stable PWM sensorless starting without any starting failure such as oscillation, loss of synchronism and reverse rotation and shorten starting time.